1. Technical Field
This invention relates generally to a frequency selective surface (FSS) and, more particularly, to a frequency selective surface filter for passing and rejecting signals in multiple selected frequency bands and for use in connection with an antenna.
2. Discussion
Frequency selective surfaces have been used in connection with wireless transmission systems such as antenna systems to reject the transmission of signals in a selected frequency band, while allowing signals in a selected frequency band to pass through the frequency selective surface. Accordingly, the frequency selective surface can advantageously be used to filter out signals at a certain frequency. Frequency selective surfaces are especially useful for satellite antenna systems where multiple signals at different frequencies may be present and only selected frequency signals are to be transmitted to or from a given antenna system device.
Known frequency selective surfaces have generally consisted of an array of conductive elements fabricated on a dielectric medium. The dielectric medium is generally transparent to signal radiation, while the conductive elements are configured to selectively allow signals of certain frequencies to pass therethrough and reject signals at other frequencies. Typically, the conductive elements are often configured as closed loops, usually configured as square loops or circular loops. Generally speaking, the dimensions of the conductive elements determine the passband and rejection band of the frequency selective surface. The use of an array of conventional single conductive loops of identical size and shape will provide a single narrow band of rejection. However, the single loop configuration provides only limited signal rejection in a rather narrow frequency rejection band.
More recently, a double-loop frequency selective surface has been used in connection with a dual reflector antenna. One example of such a double-loop frequency selective surface is described in U.S. Pat. No. 5,373,302, entitled "Double-Loop Frequency Selective Surfaces For Multi Frequency Division Multiplexing in a Dual Reflector Antenna", issued to Wu on Dec. 13, 1994. The aforementioned issued U.S. patent is incorporated herein by reference. The double-loop frequency selective surface configuration provides an array of two different size conductive loop elements on a sub-reflector which reflect signals at two different frequency bands back into a main reflector. While dual frequency reflection bands are obtainable, each of the reflection bands effectively reflects signals over a narrow range of frequencies.
In more recent times, it has become desirable to provide signal filtering for antenna operations. The multibeam phased array antenna has been developed especially for use on a satellite system which can be operable at various operating frequencies. For example, in a multiband communication system, a transmit antenna may be operable to transmit signals at frequencies in the K-band such as 20.2 to 21.2 GHz, while a receive antenna may be operable to receive signals at frequencies in the Q-band such as 41 GHz. Further, crosslink communication among satellites may operate at frequencies in the V-band such as 62.6 GHz. One problem that may arise with the transmit antenna is that the antenna's transmit circuitry generally employs power amplifiers which exhibit non-linear characteristics. These non-linear power amplifiers as well as other non-linear circuitry which are commonly provided in active antennas may produce high frequency second and third harmonics. The high frequency second and third harmonics generated by the transmit antenna can interfere with the receive and crosslink channels, unless adequate signal filtering is provided. Such a filtering device for spaceborne satellite systems and the like is generally required to be small and as lightweight as possible.
It is therefore desirable to provide for a frequency selective surface that provides both signal passing in a specified frequency band and signal rejection in multiple frequency rejection bands. It is also desirable to provide for such a frequency selective surface that realizes wide bandwidth frequency rejection. It is further desirable to provide for a frequency selective surface for use with an active antenna. It is particularly desirable to provide such a frequency selective surface filter for filtering out unwanted signals caused by the amplifier's high frequency harmonics, especially with a transmit antenna. Yet, it is further desirable to provide a frequency selective surface with multiple frequency rejection bands in a compact, low cost and lightweight package suitable for use on a spaceborne or ground antenna system.